1. Field of the Invention
The present invention pertains to a sheet decurling apparatus, and more particularly to a sheet decurling apparatus that removes the curling of sheets of paper caused by the thermal fixing of the toner image onto the sheet in image forming apparatuses such as copying machines, printers and facsimile machines.
2. Description of the Related Art
Conventionally, a sheet decurling apparatus of this kind has multiple conveyance paths and a fixed conveyance path alternating means that alternates the conveyance paths from one to the other depending on the degree of curling of the sheet, such that it automatically guides sheets that are curled to a prescribed degree or more at their top edges to a decurling conveyor path and sheets that are not curled to the prescribed degree to a non-decurling conveyance path, as disclosed in U.S. Pat. No. 5,300,012, for example. Technologies in which the degree and direction of the curling are detected by sensors and the appropriate conveyance path is selected accordingly, or in which the device is equipped with a decurling mechanism capable of varying the amount of decurling to ensure appropriate decurling have also been proposed.
Other technologies also exist in which the curling is removed by creating a loop in the sheet by stopping the rollers that convey the sheet for a prescribed period of time, as disclosed in Japanese Laid-Open Publication 1-167164, or in which appropriate decurling is attempted by detecting the degree of curling by a sensor and varying the speeds of the rollers, as disclosed in Japanese Laid-Open Publication 4-173655.
However, in the method disclosed in U.S. Pat. No. 5,300,012, the selection of a conveyance path depends on the degree of curling at the top edge of the sheet, and therefore, even when a sheet that is not curled is flush with the upper guide plate, the decurling path is selected, resulting in a lack of accuracy. In addition, the direction of curling varies: some sheets curl along the direction of conveyance and some curl in the direction perpendicular to the direction of conveyance, depending on the direction of the grain of the paper. Consequently, using the method in which the degree and direction of curling are detected, it is difficult to accurately detect the degree of curling in both directions. Even if it could be done, this method would inevitably entail an increase in cost, and is therefore impractical.
Moreover, the sheet decurling device is set such that the curling of sheets of plain paper, which is used in most cases, may be appropriately removed, presenting the ancillary effect that when thick paper or transparencies pass through, the reverse effect occurs, resulting in large curling in the opposite direction.
In the technology disclosed in Japanese Laid-Open Publication 1-167164, while the amount of loop formed at the rear edge of the sheet is constant, the degree of curling that occurs through the thermal fixing process varies depending on the properties of the sheet. In addition, the degree of curling tends to increase as the number of sheets that pass through the apparatus increases in continuous printing mode, or under similar conditions. This results in the problem that the decurling effect is either insufficient if the degree of decurling is constant or excessive where the sheet curls in the opposite direction.
The technology disclosed in Japanese Laid-Open Publication 4-173655 detects the degree of curling at the top edge of the sheet by multiple transmission sensors, but the problem exists that the probability of erroneous detection can be high depending on the behavior of the sheet while it is passing through the apparatus.